Fuel

Hard warm start low long-term fuel trim (mixture too rich) DTC. How to diagnose?

Hard warm start   low long-term fuel trim (mixture too rich) DTC. How to diagnose?
  • 2926
  • Dennis Rose
  1. How do you diagnose fuel trim?
  2. Why is my long term fuel trim high?
  3. What causes rich fuel trim?
  4. What should fuel trim be at idle?
  5. What should Ltft be at idle?
  6. How do I reset my long term fuel trim?
  7. What causes negative long term fuel trim?
  8. What causes negative fuel trim at idle?
  9. How do you know if your car is running rich?
  10. What is short-term fuel trim bank1?
  11. What controls short fuel trim?

How do you diagnose fuel trim?

Use Fuel Trim to Diagnose Vacuum and Fuel Delivery Leaks. With the engine idling, look at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values. Normal range may be high as plus or minus 8, but closer to zero is best. If the numbers are +10 or higher for STFT and LTFT, your engine is running LEAN.

Why is my long term fuel trim high?

If the engine uses a mass airflow sensor (MAF), high fuel trims at idle are a classic symptom of a vacuum leak, especially if LTFT decreases at higher engine speeds. ... A dirty or faulty MAF sensor can also cause positive fuel trim numbers because it “under-reports” airflow, driving the base air/fuel calculation lean.

What causes rich fuel trim?

This is when there are just enough parts of air (14.7) to burn one part of fuel with no excess oxygen or fuel left over. Faulty components such as dripping fuel injectors, fuel pressure regulators, ECT's and stuck open thermostats cause this problem.

What should fuel trim be at idle?

Note though that since sudden changes in the engine speed can cause short-term fuel trim values to fluctuate wildly, all fuel trim values should be taken at a minimum of at least three steady engine speeds, these being at idle, at about 2500 RPM, and at about 3500 RPM.

What should Ltft be at idle?

If long-term fuel trim (LTFT) for bank 1 is 25% at idle, but corrects to 4% at 1,500 and 2,500 rpm, focus on factors that cause a lean condition at idle, such as a vacuum leak. ... A low reading could indicate a vacuum leak or possibly a leaking EGR valve.

How do I reset my long term fuel trim?

To perform a PCM reset...

  1. Bring car up to normal operating temp.
  2. Turn ignition off.
  3. Remove ECM main fuse.
  4. Turn ignition to "on", do not start.
  5. Wait for system check to complete, numerous errors will be detected.
  6. Turn ignition off.
  7. Replace fuse.
  8. Turn ignition to "on" and wait for system check.

What causes negative long term fuel trim?

A negative long-term fuel trim means that the ECU has to decrease the injector pulse because the car is running rich. The vehicle needs less petrol in the blend.

What causes negative fuel trim at idle?

During idle when the throttle plate is closed, the vacuum in the intake manifold is high and very little air flow is entering the engine, so even a small amount of un-metered air will have an effect on fuel trim. ... This causes the fuel trim numbers to become negative during a boost situation.

How do you know if your car is running rich?

When a car is running rich, it means the engine is receiving too much fuel and too little air. Your car will still crank and drive if it's running rich, but you'll likely experience symptoms like low gas mileage, slow acceleration and strong smell of gasoline (especially when idling).

What is short-term fuel trim bank1?

The short-term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. ... The long-term fuel trims (LTFT) are driven by the short-term fuel trims. LTFT refers to changes in STFT but averaged over a longer period of time.

What controls short fuel trim?

Short-term fuel trim happens as a result of changes in the oxygen levels of exhaust gases. An oxygen sensor monitors the flow and generates a signal that is proportional to the oxygen levels in the gases.

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